The Comparative Pathobiology Group has focused much of its research on defining the pathogenesis of disorders affecting the reproductive tract of humans and rodents and assessing the role of environmental, endogenous and growth factors on the growth and induction of these disorders. In understating the role of proliferation in human uterine leiomyoma (fibroid) growth, we have found that both positive and negative regulators of apoptosis are not differentially expressed in fibroids compared to normal myometria, and that altered apoptosis does not appear to play a significant role in the development of these tumors. Our studies show that cell proliferation may be the most significant contributor to fibroid growth, although, it appears to be phasic, does not correlate with tumor size, and is autonomous for each tumor within a uterus. In studies addressing the role of growth factors in the pathogenesis of fibroids, we have found that IGF-I is overexpressed in fibroids compared to normal myometria during the proliferative phase of the menstrual cycle and that the IGF-I receptor is activated. These studies will help to define some of the basic biological and molecular pathways important in fibroid growth, which can then be applied to developing alternative noninvasive treatment regimens for fibroids. In vitro model systems for studying fibroids are limited in that human derived leiomyoma cells grow poorly in culture. We have overcome this obstacle by development of hTERT (human telomerase) immortalized uterine leiomyoma and myometrial cell lines. These cells are being used to study leiomyoma tumorigenesis in a prospective manner. In determining the role of environmental agents in fibroid development we have found that in CD-1 mice prenatal and neonatal exposures to diethylstilbestrol (DES) results in uterine leiomyomas similar to fibroids observed in women. Also, the phytoestrogen, genistein, can be stimulatory or inhibitory to uterine leiomyoma cell growth depending on its concentration.